A fluorescent display device in which a second display is arranged on a light permeable substrate of an envelope of a conventional fluorescent display tube to display a predetermined pattern by illuminating the second display with an outer light source is proposed in Japanese Patent Publication S55-102162.
The fluorescent display device having the conventional second display will be explained with reference to FIG. 5. In the drawings, FIG. 5A is a cross-sectional view of the fluorescent display, and FIG. 5B is a plane view showing a display area of the fluorescent display in which a front plate is omitted. As shown in FIG. 5A, the fluorescent display has an envelope having a glass or anode substrate 81 on which an anode A including a wiring 91 such as an anode wiring, and an insulation layer or film 84, a glass front plate 82 and side plates 83 are arranged. The anode A includes a phosphor film or layer 93 and an anode electrode 92. The phosphor film 93 is deposited on the anode electrode 92. The anode electrode 92 is connected to the anode wiring 91 with a conductor via a through hole of the insulation layer 84. The phosphor film 93 emits light by an electron generated by a filamentous electron source F.
In the insulation layer 84, a second or outer light source display 841 forming a pattern such as characters or symbols is arranged by removing a part of the insulation layer. In the outer surface of the envelope, a lump LP illuminating the second display 841 is arranged. The fluorescent display device shown in FIG. 5A is observed from the arrow X0. The display area of the fluorescent display device in FIG. 5A, as shown in FIG. 5B, includes first fluorescent displays Dp1 and Dp2 displaying a predetermined pattern by the anode A and a second outer light source display Dp3 displaying a predetermined pattern by light of the lump LP.
On the outer light source display 841 of the conventional fluorescent display, a pattern such as characters or symbols is formed by removing the insulation layer 84. The insulation layer 84 is made primarily from glass and pigment. As a result, a light of the lump LP penetrates through a remaining portion or un-removed portion of the outer light source display 841, and the insulation layer 84 around the outer light source display 841. Even through a black insulation layer adding black pigment to the insulation layer is used, light shielding function is not enough. For this reason, the conventional fluorescent display device can not clearly display the character or symbol pattern of the outer light source display 841. Particularly, a high light shielding function is required in the fluorescent display device for use in a vehicle, because the leaked light from the insulation layer stands out at night or a dark place. Accordingly, an object of the present invention is to provide a fluorescent display device eliminating the conventional problems of the fluorescent display device having the outer light source display.